It is currently thought that subunit III of cytochrome oxidase plays an important role in the ability of this enzyme to pump protons. We have shown, however, that this subunit is neither required nor beneficial to this process. Using a rat liver submitochondrial system, the quantitative effect of delta-psi on energies of activation for the synthesis and hydrolysis of free ATP from free ADP and Pi was determined. It was found that approximately 83% of the protonmotive input energy was used to lower the energy of activation for synthesis and approximately 17% to raise the energy of activation for hydrolysis.